Mobile, in particular wireless communication is broadly used in several fields of applications, e.g. in the home, public or office area, as well as for any kind of communication, e.g. speech, data, and/or multi-media communication. Basically, such applications are confronted with two major problems.
As a matter of fact, available bandwidth for transmitting information is limited due to the general shortage of available spectrum. In this regard, it is well known to modulate both the amplitude and the phase of the carrier to reduce required bandwidth. For instance, systems, in which the amplitude and the phase are modulated, i.e. which process wide-band complex envelope signals, are EDGE, UMTS (WCDMA), HSxPA, WiMAX (OFDM) and 3G-LTE (OFDM). However, amplifying amplitude modulated carriers without distortion in the transmitter output stage imposes significant linearity constraints on the output stage amplifier.
Further, power efficiency of mobile transmitters is important, since mobile terminals in wireless communication are typical portable and as such, usually battery powered. In mobile terminals, the output stage of the transmitter unit is usually the largest power consumer of the whole device. Consequently, any improvement in this stage with respect to power efficiency is appreciated. Known efficient power amplifiers topologies or circuit arrangements are, for instance, class-C and class-E radio frequency (RF) amplifiers in which the active output amplification devices conduct current only at the time, e.g. in case of transistors, when the collector-emitter voltage is at its lowest value. Unfortunately, class-C and class-E amplifiers are very nonlinear and thus, introduce substantial distortion of the amplitude modulation.
Linear Amplification using Nonlinear Components (LINC) is a well-known concept for high efficient linear power amplification of RF signals. Detailed information may, for instance, be gathered from S. C. Crips, “Advanced Techniques in RF Power Amplifiers Design”, Artech House 2002, or from D. C. Cox in “Linear Amplification with Nonlinear Components”, IEEE Transactions on Communications, December 1974, pp. 1942-1945.
WO 01/45205 discloses transmission of multiple radio channel frequency signals modulated with respective information modulation from a common antenna at multiple radio frequencies. Multiple modulators are provided, a respective one of which corresponds to a respective one of the radio channel frequencies. Each modulator generates at least one constant amplitude, phase modulated drive signal at the corresponding radio channel frequency from the respective information modulation, such that the at least one constant amplitude, phase modulated drive signal corresponds to the information modulation for the corresponding radio frequency. At least one saturated power amplifier is provided for each of the at least one constant amplitude, phase modulated drive signals. A respective saturated power amplifier is responsive to the corresponding constant amplitude, phase modulated drive signal, to produce a corresponding amplified output signal at an output thereof. A coupling network connects the outputs of the saturated power amplifiers in series, to produce a combined signal that is applied to the common antenna, such that the common antenna radiates the radio channel frequency signals that are modulated with the respective information modulation. In some embodiments, at least two constant amplitude phase modulated drive signals are provided at the corresponding radio channel frequency, such that the at least two constant amplitude, phase modulated drive signals correspond to the information modulation for the corresponding radio frequency.
The LINC concept, also known as out-phasing, is illustrated by means of the simplified out-phasing power amplifier (PA) 100 shown in FIG. 1. Accordingly, an amplitude (A(t)) and phase (φ(t)) modulated RF signal Sin(t), represented by equation (1), is split by means of a signal component separation unit 102 in two RF signals S1(t), represented by equation (2), and S2(t), represented by equation (3), each being phase modulated by the baseband phase information φ(t) and an out-phasing angle θ(t) that is based on the baseband amplitude information A(t), according to equation (4). The most important aspect is the fact that both RF signals S1(t) and S2(t) have constant amplitude.Sin(t)=A(t)sin(ωt+φ(t))  (1)S1(t)=½ sin(ωt+φ(t)+θ(t))  (2)S2(t)=½ sin(ωt+φ(t)−θ(t))  (3)θ(t)=arcos(A(t))  (4)
Then, the signals S1(t) and S2(t) with constant amplitudes can be separately amplified by means of efficient nonlinear saturated power amplifiers RF PA1 and RF PA2 in amplification branches 110, 120. After amplification, the output RF signal can be reconstructed by means of a signal component combiner unit 104. The output signal of the combiner unit equals the sum (or difference) of the two input signals S1(t) and S2(t) as depicted by equation (5).Sout=S′1(t)+S′2(t)=G cos(θ(t))sin(ωt+φ(t))=G A(t)sin(ωt+φ(t))  (5),where G represents the gain of the amplification stages, i.e. the power amplifiers RF PA1 and RF PA2.
Ideally voltage sources are to be combined so that the average current in the amplification devices can vary as function of the out-phasing angle θ(t). However, if ideal class-A, class-B or class-C operation is applied, the amplification devices act as current sources and the DC current does not vary with the out-phasing angle, meaning that the efficiency drops linearly with output power, i.e. class-A like. However, in overdriven or saturated class-A, class-B or class-C operation modes, the amplification devices act more as voltage sources. That is, approximately independent of input drive and output current and the DC current is able to vary with the out-phasing angle. Ideally the efficiency will drop according to the square root of the output power, i.e. class-B like. So effectively there is no gain in power efficiency compared to a linear class-B PA design.
Power efficiency of a LINC amplifier depends strongly upon the type of power combiner used at the output. The efficiency of the combiner is reduced by the reactive part of the impedance. One combining technique that circumvents such losses is the well known Chireix combining technique as described in H. Chireix, “High power out phasing modulation”, Proceedings of the Institute of Radio Engineers (Proc. IRE), vol. 23, no. 11, pp. 1370-1392, November 1935. Accordingly, the reactive part of the effective load impedance can be cancelled by the use of compensating reactances. However, in practice it is difficult to improve the efficiency for a wide range of output power levels by implementing reactive cancellation, e.g. by using a so called RF Micro-Electromechanical System (MEMS) switches for tuning the compensating reactance as function of output power in discrete steps.
Another way to optimize the efficiency is to use switching-mode PAs, e.g. operated in class D, E, DE, or F mode, in an out-phasing configuration. In a class-DE out-phasing power amplifier with variable duty cycle the duty cycle can be used on each out-phasing path to compensate for the losses generated by the combining network, i.e. instead of the afore-mentioned Chireix component. However, this technique relies on accurate phase and duty cycle generation as well as control of the driving signals. Importantly, this offers very desirable capabilities for software-defined-radio (SDR) applications, where the duty cycle could be used in a digital calibration and/or pre-distortion routine during start-up of the radio for efficient and linear operation of the transmitter.
As mentioned above reduction of power consumption in transmitter circuits, in particular in the power amplifier circuitry therein, is very important. One method to realize desired reduction is use switching power amplifiers (PA) having better efficiency. However, switching PA concepts are only feasible in combination with suitable modulation methods like pulse width modulation (PWM) and out-phasing concepts, which in turn rely on accurate phase control and the duty cycle of the signals. Accordingly, one of the problems in the connection with switch-mode out-phasing PA concept is the generation of the required phase and duty cycle modulated signals with suitable accuracy.